Calorimetric Studies by Differential Thermal Analysis
1
EDWARD M. BARRALL II, ROGER S. PORTER, and JULIAN F. JOHNSON California Research Corp., Richmond, Calif.
b A differential thermal analysis cell has been developed which permits precision calorimetry on polymer and inorganic samples irrespective of initial aggregation state. The apparatus is contrasted with differential thermal analysis cells of conventional design using centrally located thermocouples. Calorimetric errors which arise from changes in sample thermal conductivity are demonstrated to exist in cells of conventional design and to b e almost totally removed by the new cell design. The effect of using the new cell is illustrated by tests on a polymer sample with two fusion points and on blends of ethylene and propylene polymers. The necessity of calibrating differential thermal analysis cells at more than one temperature for calorimetry is demonstrated.
vide a reproducible system but in itself changes the thermal history of the sample. Moreover, it is frequently desirable to determine heats and transition temperatures of materials as received, not after some conditioning pretreatment. I n some cases-e.g., block copolymers and homopolymer mixtures-two fusions are noted. Both effective thermal conductivity and heat capacity of the polymer change after the first fusion peak. Thus, it is impossible to relate the magnitude of the heat of the second fusion to that of the first fusion by simply using chart areas by the usual methods. For these reasons, it is difficult to calibrate a conventional DTX accurately for calorimetry, particularly of polymers.
D
Construction of Apparatus. Herold and Planje (6) suggested the use of a thermocouple as the sample cell to simplify the heat transfer problem.
thermal analysis (DTA) has been shown to have the potential for calorimetric studies. Several authors have shown that the size and shape of test samples (chuuk, pellet, powder, large granules, etc.) can have a great effect on the apparent size, shape, and temperature of simple first-order transitioii peaks as seen by DTA (3, 9, I S ) . From a thermodynamic point of view, state of aggregation should not influence the amount of heat required for a solid-solid or solid-liquid phase transition. However, when a thermocouple is placed within a sample, the eficiency of heat transfer through the sample becomes a major factor determining the size and shape of the DTh peak, particularly in determining the heat of fusion of polymers. Consecutive thermograms on the same sample are frequently different, irrespective of any crystalline changes which may have occurred on the first heating. The principal factor here is that the thermal conductivity across a powder is different from the conductivity across a fused mass. To overcome this variable, authors have resorted to diluting the sample with inert material (solid or liquid) ( 3 , 4))or fusing the sample in contact with the measuring thermocouple (6, 8, 12) prior to analysis. Dilution of the sample to overcome the gross ponder effects is unsatisfactory except in restricted cases. Fusion of the sample to the thermocouple can pro1 Part I1 of a series on Thermo-analytical Techniques IFFERENTIAL
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ANALYTICAL CHEMISTRY
EXPERIMENTAL
Figure 1 .
Calorimeter DTA cell, block
A A,B.
C a p p e r sample cups, 4-mm. 0.d. b y 6 mm. C. Copper reference cup D. Two-conductor ceramic supports, 3-mm. diameter b y 50 mm. E. Copper radiator shield, 35-mm. diameter b y 5 3 mm. F. Program-sensing thermocouple G. liquid COz cooling gas jet H . Electric furnace, 45-mm. i.d. b y 100 cm. 1. C o p p e r base plate, 38-mm. diameter Outer base plate, Transite sheet with a raised ring for accurate furnace centering Whole apparatus placed in bell jar for otmosphere control, Sample cup lids not shown
Their construction consisted of a bimetallic vessel, which leads t,o several difficulties in fabrication. To overcome these construction difficulties, block -4, Figure 1, was developed. A similar apparatus has been described by \T'hit,e and Koyama for the measurement of stored-energy release in metals (14). Helium was used as a conducting gas with some loss in efficiency due t,o nonadiabatic conditions. Their work adequately demonstrated the advantages of this method for the DT.4 of conduct'ing met'al powders and solid samples. In the present work No. 27 gage Leeds and Northrup duplex copperconstantan thermocouples were silversoldered to a 4-mm. 0.d. copper sample cup fitted with a copper lid. The thermocouples and sample cups were supported on ceramic insulators 50 mm. in height. The insulators were held in a machined base plate, I , as shown in Figure 1. The cell array was covered by a blackened metal shield to which the program control thermocouple was connected. The shield, E , was programmed by the temperature controller, previously described @), using the furnace, H , as a heat source. The cells were heated only by thermal radiation received from the blackened copper radiator shield. By using a radiator, radiation hot spots which may be due to the furnace windings were eliminat,ed. The DT.4 cell was operated in a bell jar (not shown in Figure I ) , which provided a controlled atmosphere-from vacuum t'o 2 at,m. with a choice of gas. Individual cups were provided for two samples and a reference material. The second sample cup is used when two samples with different transition temperatures are to be thermographed. By external manipulat,ion of the thermocouple connections, two thermograms may be obt,ained in one heating program without dishrbing the apparatus. The thermal isolation of the sample from possible heat loss or gain by conduction from sources not measured by the thermocouple is minimized. The absolute and differential temperatures of the sample are measured with the same thermocouple. The temperature measuring and programming system has been described (2). Procedure. Baker reagent-grade ammonium nitrate was used as a standard inorganic reference material. .\lthough the DT.1 behavior of this material is somewhat erratic below 100' C., the endotherms a t 128" and 168" C. we relatively reproducible (10, 1 3 ) . .I misture of 49y0 polyethylene-
a001
PO
UO
140
IM
IW
200
220
Block A was calibrated by measuring the endothermal peak area generated by ammonium bromide, 137.2' C., 882 cal. per mole (1); ammonium chloride, 183.1' C., 1873 cal. per mole ( 1 ) ; potassium thiocyanate, 177.0' C., 2500 cal. per mole (11); and benzoic acid, 121.8' C., 4140 cal. per mole (11) at six heating rates. This produced a group of lines, one for each sample, which passed through zero area a t zero heating rate. Figure 2 was obtained by plotting calories per unit area vs. temperature of the endothermal minimum a t constant heating rate. To find the number of calories involved in any process, the area of the DTA peak obtained a t a given heating rate is multiplied by the calories per unit area value taken from Figure 2 a t the same heating rate, and a t the temperature of the peak under consideration.
1
240
TEMPERATURE OF DTA PEAK,%
RESULTS
The performance of block A approaches adiabatic calorimetry, as shown by the thermogram of ammonium nitrate in Figure 3. Curve 1 was obtained in the conventional manner of DTA with the thermocouple directly in contact with a 0.0260-gram sample of ammonium nitrate diluted to 30 weight % ' with Carborundum as previously described ( 3 ) . The peak a t 128' C. in Figure 3 is known to require 12 cal. per gram, and the fusion a t 168' C. requires 19 cal. per gram (10). The areas in curve 1 of Figure 3 do not indicate the proper relative heats of transition because of the large change in thermal conductivity of the sample a t the melting point (168' C.). This affects the magnitude of the temperature change measured a t the thermocouple located in the center of the sample. The heats involved a t 128' and 168' C. calculated from curve 2 of Figure 3 and the calibrations shown in Figure 2 are 11.8 and 19.1 cal. per gram, respectively, in good agreement with literature values. Since the copper cup is essentially the only thermal reservoir or source of heat for the sample, its temperature will more accurately reflect the total caloric process occurring a t any spot in the sample in a given temperature interval than with centrally located thermocouples. Indeed, no change in peak areas was observed with block A when the sample aggregation was changed-i.e., powder to fused mass. The data in Figure 2 are further evi'dence that the calorimetric DTA cell effectively removes variations in peak area due to physical aggregation and type of transformation. Benzoic acid and potassium thiocyanate represent fusions-Le., solid-liquid transitions. Ammonium bromide and chloride undergo a solid-solid transformation. Using conventional DTA arrangements with the sample in contact with a centrally
Figure 2. Variation of calories per unit area in DTA peak with temperature and heating rate, block A
polypropylene powder was used as a typical sample with two fusion endotherms. The fusion characteristics are very similar to those of the corresponding block copolymer (7). For comparison 30y0 diluted samples were made with Carborundum as described ( 3 )'
The samples used in block A were weighed directly, without dilution, onto a 1-em. square aluminum foil, which was carefully folded and placed in the sample cup. An equal foil weight was used in the reference cup. Conventional differential thermograms were obtained using block B (2). Calibration. The conventional D T A , block B , was calibrated according to the method of Barrall and Rogers ( 3 ) using ammonium chloride and bromide. A plot of peak area os. heating rate using samples with known heats of fusion was obtained. The straight line generated by these data did not pass through zero heating rate.
SAMPLE TEYPERATURE,'C.
Figure 4. Dual fusion differential thermograms made in block 8 Thermograms of polyethylene, polypropyl ene, and physical mixture containing 49% polyethylene, 51 polypropylene run a t heating r a t e of 7.0' C./minute
YO
I MIXTURE
%
-
POLYETHYLENE POLYPROPYLENE
1 99
I I28
I68
SAMPLE TEMPERATURE,*C.
Figure 3.
0
Differential thermograms of ammonium nitrate Run 1 , block 8 Run 2, block A
Active sample weight 0.0080 gram, heating r a t e 4.7' C./rninute
I
50
I00
I50
I
I
200
250
JOO
SAMPLE TEMPERATURE,'C.
Figure 5. block A
Dual fusion differential thermograms made in
Thermograms of polyethylene, polypropylene, and physical mixture containing 49% polyethylene and 51% polypropylene run a t heating r a t e of 7.0' C./minute
VOL. 36, NO. 1 1 , OCTOBER 1964
2173
located thermocouple, the relationship between endothermal area and calorie intake for fusions and solid state transformations is very different. The gentle slope of the lines in Figure 2 results from the heat capacity-temperature relationship of the apparatus. This implies that more heat is required to change the cell temperatures 1’ a t 200’ C. than a t 100’ C.-in perfect agreement with the usual thermodynamic considerations. This demonstrates the frequently overlooked necessity of calibrating any DTA apparatus a t two temperatures or more for calorimetry. Figures 4 and 5 show thermograms of a 49% polyethylene51% polypropylene mixture and of corresponding amounts of the unmixed pure polymers. These are the powdered polymer samples from which the mixture was made. Irrespective of any crystalline effects, the two endotherms seen in the thermogram of the mixture should have a total area equal to the sum of the areas given by the unmixed polymers. This is not the case when the thermocouple is mounted conventionally in the center of the sample (Figure 4). The polypropylene endotherm is somewhat smaller in the pure polymer than in the mixture because of the higher thermal conductivity of the mixed sample at the polypropylene transition in the presence of fused polyethylene. The fused polyethylene conducts heat better than either
of the powdered polymers alone. Better thermal contact between the centrally located thermocouple and the total sample exists after the polyethylene fusion. Therefore, more of the heat absorbed by the polypropylene transition is sensed a t the thermocouple. The heat absorbed when polypropylene alone is present is not fully detected a t the central thermocouple. I n Figure 5 the same experiment is shown conducted in block A . The sum of the area of the individual polymer samples is equal to within 5% of the total area of the mixed sample. The endotherms are somewhat broader and not as sharply defined because of the integrating effect of the copper cups. Block A significantly reduces the heat capacity and transfer problems involved in polymer DTA. However, these effects cannot be totally removed as long as the sample size remains finite. The shift in base line before and after the fusions more accurately reflects the true change in heat capacity of the sample before and after fusion. When the mixture is heated a t a rate less than 2” C. per minute, some merging of the endotherms is noted due to the dissolution of polypropylene in molten polyethylene.
mer DTA and Dan Trujillo for carrying out part of the experimental work. LITERATURE CITED
(1) Arell, A., Ann. Bcad. Sci. Fennzcae,
Ser. A VI, No. 57, 42 (1960). M.,11, Gernert, J., Porter, R. S., Johnson, J. F., ANAL. CHEM. 35, 1837 (1963). ( 3 ) Barrall, E. M., 11, Rogers, L. B., I b i d . , 34, 1101 (1962). (4) Borchardt, H. J., Daniels, F., J . Am, Chem. Soc. 79. 41 119571. ( 5 ) Clampitt, B. H:, A ~ A L . CHEM. 35, ( 2 ) Barrall, E.
577 (1963). (6) Herold, P. G., Planje, T. J., J . Am. Ceram. Soc. 31, 20 (1948). (7) Ke, Bacon, J . Polymer Sci. 61, 47 i1962). ( S j Ke,’ Bacon, “Organic Analysis,” J. Mitchell, Jr., ed., T’ol. IV, Interscience, New York, 1960. (9) MacKenzie, R. C., “Differential Thermal Investigation of Clays,” AIineralogical Society, London, 1957. (10) O’Xeill, M .J., Zbid., 36, 1238 (1964). (11) Rossini, F. D., Wagman, D. D., Evans, W. H., Levine, S., Jaffe, I,, “Selected Values of Chemical Thermodynamic Properties,’’ Nat. Bur. Stand. Circ. 500, (1952). (12) Vassallo, D. A., Harden, J . C., AN.AL.CHEM.34, 132 (1962). (13) Watson, E. S., O’Keill, M . J., Justin, J., Brenner, S . , Ibid., 36, 1233 (1964). (14)White, J. L., Koyama, K., Rev. Sci. Instr. 34, 1104 (1963).
ACKNOWLEDGMENT
We thank L. B. Rogers for the initial suggestion, made in 1959, that such a cell arrangement could be of benefit in poly-
RECEIVED for review February 21, 1964. Accepted May 11, 1964. Division of Analytical Chemistry, 148th Meeting, ACS, Chicago, Ill., September 1964.
High Resolution Nuclear Magnetic Resonance Analysis of Vinyl Acetate Copolymers MARTIN W. DIETRICH and ROBERT E. KELLER Research Department, Organic Chemicals Division, Monsanto Co., St. louis, Mo. 63 7 77
b A rapid high resolution NMR method has been developed for the quantitative determination of the composition of vinyl acetate copolymers. The dibutyl fumarate, dibutyl maleate, dioctyl maleate, or 2-ethylhexyl acrylate content of vinyl acetate copolymers is determined by peak height measurement of proton signals specific for the comonomer and for vinyl acetate. The concentration of reactant is determined from a calibration curve prepared from copolymers of known composition. The method is estimated to be accurate to within 2 mole absolute in the range of 0 to 20 mole % comonomer.
70
P
INVOLVING the determination of the composition of copolymeri have received considerable attention in recent years. Several analytical techniques have been utilized ROBLFNS
2 174 *
ANALYTICAL CHEMISTRY
for studies of this type. The techniques most generally used involve either pyrolysis of the polymer with gas chromatographic analysis of thP fragments (5)or infrared absorption analysis of the polymer per se (3). These techniques are often inaccurate and/or time consuming. With the pyrolysisgas chromatography scheme, the pyrolysis mode and proper operating conditions must be determined; with infrared analysis, the preparation of satisfactory thin films is often difficult. Quantitative high-resolution NMR spectroscopy offers a convenient alternative approach to the analysis of copolymers. Two main requirements must be satisfied in order for NMR analysis to be feasible; the polymer must have sufficient solubility in an appropriate solvent and it must produce S Z I R spectral peaks narrow enough to permit differentiation between comono-
mers. These requirements place limitations on the versatility of the technique for different polymeric materials. When applicable, however, NMR is a rapid nondestructive method for the analysis of copolymers (6,6, 7 ) . The vinyl acetate copolymers studied in this work represent additional examples of successful application of the NMR method. EXPERIMENTAL
The N M R spectra were obtained with a Varian Associates HR-60 high-resolution S M R spectrometer equipped with a V-3521 integrator. Spectra were recorded on a Mosely Model 3-S X Y recorder a t a sweep rate of 7.3 c.p.s./second a t an R.F. level below saturation. Scale calibration was determined by the method of modulation side bands. Copolymer samples for use as standards were prepared as latex emulsions (4). .Is used, this technique is ex-
